In various therapies, e.g. in hemodialysis, it is required that the blood pressure of the patient is continuously monitored. For this purpose, use is made of devices for indirect (non-invasive) blood-pressure measurement with the aid of a cuff to be laid around the patient's arm, which cuff will be inflated and then will be slowly deflated while a pressure measurement process will be performed at the same time. For achieving a quasi-continuous blood-pressure control, the control means has to perform control processes in intervals of a few minutes. The number of blood-pressure measurements required for such a control process during a treatment procedure covering several hours can be considerably reduced if, at predetermined points of time, blood-pressure measurements are substituted by blood-pressure values which were obtained in earlier treatment procedures performed on the same patient. U.S. Pat. No. 6,578,241 B2 and EP 1 226 838 A2 describe corresponding therapy devices with interval-based blood-pressure measurement and a reduced number of measurement processes. Herein, on the basis of an identical length of all intervals, e.g. five minutes per interval, the control means safeguards that the blood-pressure development and the blood-pressure trends will be evaluated in each interval in accordance with uniform rules, as is described in EP 0 956 872 A2.